1. Field of the Invention
The present invention relates to a battery protection circuit, a method for protecting a battery, a power supply device, and a program thereof. 
2. Description of Related Art
A battery protection circuit that protects against degradation of a battery caused by overcharge and over discharge of a power supply device with a built-in rechargeable secondary battery, such as a battery pack described in Japanese patent number 3382002 (hereafter patent reference 1), is widely known.
For example, the battery protection circuit described in patent reference 1 stops a battery from discharging immediately after (500 milliseconds) an alarm when over discharge of the battery progresses. Similarly, when overcharge of the battery progresses, charging of the battery is stopped immediately after the alarm.
In the battery protection circuit described in patent reference 1, when the overcharge or over discharge progresses, a discharge path or a charge path is cut off in order to protect the battery. At this time, when it is in an over discharge condition, the battery recovers from the over discharge condition through charging of the battery by a user and becomes reusable. On the other hand, when the user leaves an over discharge condition of the battery and does not recharge, over discharge further progresses. When the over discharge progresses, eventually, the battery is degraded until it is in a nonreusable condition.
When the battery is, for example, a lithium-ion battery, due to the over discharge, cobalt of a positive electrode is eluted, and copper of a collector of a negative electrode is eluted. As a result, the lithium-ion battery does not function as a secondary battery. Furthermore, it is well known that the over discharge of the lithium-ion battery could cause abnormal heating of the battery.
In contrast, the condition in which a user does not recharge even though the battery is in the over discharge condition is not assumed in the battery protection circuit of patent reference 1. In other words, the battery and an over discharge detecting part (or overcharge detecting part) are constantly connected to each other in the battery protection circuit of patent reference 1. Under this structure, the battery keeps supplying power with respect to the over discharge detecting part (or overcharge detecting part) even though the over discharge progresses up to the point in which the battery is nonreusable.
Accordingly, it is effective to extend a period of time to fall in a condition in which abnormal heating due to over discharge occurs by eliminating all of the elements that progress the over discharge in the over discharge detecting part (or overcharge detecting part) with respect to the battery which is in the nonreusable condition due to the over discharge.